With the recent spread of personal computers and mobile terminals, digital data communication (data communication) is beginning to be widely performed across the Internet. One digital data circulated in data communication is a motion image. Since a motion image generally has a large data amount, the transmission data amount is reduced before transmission by encoding the motion image by using still images contained in the motion image and sounds attached to these still images as units.
One known motion image data communication method of this type is a method by which transmission data is given characteristics (scalability) which, as decoding of image data and sound data contained in motion image data is advanced on the receiving side, improve the quality of the decoded image or decoded sound.
To give scalability to data to be transmitted as described above, scalability is given when image data and sound data to be transmitted are encoded.
In the above conventional communication method using scalability, however, scalability is given to transmission data when the data is encoded. Therefore, to give scalability to image and sound data already encoded, it is necessary to once decode these data and again encode the data to give scalability to them.
Also, no encoding method capable of generating encoded data containing both image data and sound data while giving scalability to both the data has been established.